The DHB has two high current half bridge outputs so you drive two DC motors in one direction, or alternatively you can drive one DC motor in two directions by combining the two outputs (as per the diagram). What you are wanting to do is high speed PWM control of a single motor for the purposes of speed control (which the DHB will also do a perfect job of).

For your application you will only need one output and one input. Set up a table in the M800 to provide the duty cycle you want to the DHB controller and then connect the output to your Weldon pump. The DHB controller supplies the 12V feed to the pump and the ground for the pump needs to go to chassis ground or battery negative.

Using the DHB circumvents the need for a traditional fuel pump relay. I've attached the full Motec DHB data sheet which fully explains the product and gives a couple of alternative wiring diagrams.

Can the Motec DHB be used with other aftermarket ECU's or is it purpose built for just Motec units? I'm asking as im looking into running twin 044 pumps, but controlling them using PWM either through a SSR, or better still one of these Motec DHB's

^ This goes both ways. The DHB is NOT a cost effective way to implement simple PWM functionality that an SSR will provide. $500 is generally a tough pill to swallow when you can accomplish the same result with a couple quality (read non-knockoff) $50 relays. Food for thought, and I'm not saying MoTeC gear is anything but awesome.